Conventional surgical procedures involve cutting through bodily structures to expose a lesion or organ within the body for treatment. Because these procedures create considerable trauma to the patient, physicians have developed minimally invasive procedures using probes inserted into the body through body orifices or through small holes to treat or measure structures within the body. For example, the devices commonly referred to as endoscopes include an elongated body having a distal end and a proximal end. The distal end of the probe body can be inserted into the gastrointestinal tract through a body orifice. The endoscope may be equipped with optical devices such as cameras or fiber optics to permit observation of the tissues surrounding the distal end, and surgery may be performed by inserting and maneuvering surgical instruments through a channel in the endoscope body. Other probes commonly referred to as laparoscopes and orthoscopes are inserted into the body through small holes formed in surrounding tissues to reach the bodily structures to be treated or measured. Still other probes, commonly referred to as catheters, can be advanced through the vascular system, as through a vein or artery, or through other bodily passages such as the urinary tract.
The physician can guide the probe to the desired location within the body by feel or by continuously imaging the probe and the body, as by fluoroscopy, during the procedure. Where the probe includes optical elements, the physician can guide the probe based on visual observation of the tissues surrounding the distal tip of the probe. However, this option is available only for probes such as conventional endoscopes which are large enough to accommodate the optical elements. Moreover, optical guidance normally is useful only where the distal tip of the probe is disposed within a cavernous organ; it is not normally useful in guiding the probe within solid or semisolid tissues.
As described, for example, in U.S. Pat. Nos. 5,558,091, 5,391,199; 5,443,489; and in PCT International Publication WO 96/05768, the disclosures of which are hereby incorporated by reference herein, the position, orientation or both of the distal end of a probe can be determined by using one or more field transducers such as a Hall effect or magnetoresistive device, coil or other antenna carried on the probe, typically at or adjacent the distal end of the probe. One or more additional field transducers are disposed outside the body in an external frame of reference. The field transducers preferably are arranged to detect or transmit non-ionizing fields or field components such as a magnetic field, electromagnetic radiation or acoustical energy such as ultrasonic vibration. By transmitting the field between the external field transducers and the field transducers on the probe, characteristics of field transmission between these devices can be determined. The position and/or orientation of the sensor in the external frame of reference can then be deduced from these transmission characteristics. Because the field transducer of the probe allows determination of the position of the probe, such transducer is also referred to as a "position sensor".
As described, for example, in the aforementioned U.S. Pat. No. 5,558,091, the frame of reference of the external field transducers can be registered with the frame of reference of imaging data such as magnetic resonance imaging data, computerized axial tomographic data, or conventional x-ray image data and hence the position and orientation data derived from the system can be displayed as a representation of the probe superimposed on an image of the patient's body. The physician can use this information to guide the probe to the desired location within the patient's body, and to monitor its orientation during treatment or measurement of the body structure. This arrangement greatly enhances the ability of the physician to navigate the distal end of the probe through bodily structures. It offers significant advantages over conventional methods of navigating probes by feel alone. For instance, because it does not require acquisition of an optical image of the surrounding tissues for navigation purposes, it can be used with probes which are too small to accommodate optical elements, and can be used for navigation of the probe within solid or semisolid tissues. The transducer-based system also avoids the difficulties associated with navigation of a probe by continuous imaging of the probe and patient during the procedure. For example, it avoids exposure to ionizing radiation inherent in fluoroscopic systems.
As described in certain embodiments taught in U.S. Pat. No. 5,391,199, the system can include one or more reference catheters and a mapping/ablation catheter. Each of these catheters has a field transducer as discussed above disposed adjacent the distal end of the catheter. The mapping/ablation catheter is provided with electrodes for detecting local electrical activity at the distal end of such catheter, and for applying radio frequency energy to ablate surrounding tissue. The reference catheters may be positioned with their distal tips at fixed locations within the heart, whereas the mapping/ablation catheter can be moved within the heart while measuring electrical activity. The tip positions of the reference catheter and of the mapping/ablation catheter are monitored in the frame of reference of external antennas. Electrical activity data and positional data provided by the mapping/ablation catheter provides a map of the electrical activity of the heart. The reference catheter position information can be used to compensate for movement of the heart, and to register the mapping/ablation catheter position data with images such as fluoroscopic or MRI images. In certain procedures taught in the '199 patent, the map can be used to locate a site within the heart for treatment, and the position information provided by the position sensors can be used to maneuver the mapping/ablation catheter to the treatment site.
However, still further improvements in transducer-based probe navigation and treatment systems would be desirable. In particular, it would be desirable to provide accurate guidance of a probe without reliance on registration between the sensor-based positional data and previously-acquired image data. This would be particularly desirable for placement and treatment of a probe in relatively soft, mobile tissues such as the breasts, lungs, liver, gastrointestinal tract and other internal organs. Moreover, it would be desirable to provide a system which provides probe guidance information to the physician in a form which can be readily assimilated and used by the physician. It would also be desirable to provide a system which facilitates the use of multiple probes in combination to treat, observe or measure a body structure.